An electronic package serves as a protective enclosure for a die while permitting electrical connections between the die circuitry and the circuitry on a carrier or substrate such as a printed wiring board. The packages of interest in this invention are hollow bodies that filly enclose the die and that are initially formed as open receptacles with electrical leads or "traces" embedded in the walls. Once the receptacle is formed with the leads embedded in its walls, the die is placed inside the receptacle and electrically joined to the leads. The receptacle is then closed and sealed for further processing, including further electrical connections and use.
To achieve consistent and reliable performance with the extremely fine circuit lines and high current densities that are currently used in dies, it is very important that water vapor and other atmospheric gases be prevented from entering the package once it is sealed. By sealing the die and protecting it from exposure to these gases, the package enhances the performance of the die in humid and other potentially harmful environments. The leads that form the electrical connections between the die and the components external to the package are thin strips of metal that pass through the package walls, the inner ends of the strips bonded to the die circuitry and the outer ends available for bonding to the substrate circuitry. Since the strips and the package walls are made of dissimilar materials, it is difficult to form a secure bond between them that will serve as a vapor barrier. This difficulty is particularly acute when there are large differences in the rates of expansion and contraction of the materials. This expansion and contraction occurs in response to temperature variations that occur during the thermal cycling that the package encounters during processing steps such as die bonding, wire bonding and soldering. Temperature variations also occur in the typical environment in which the package used also stem from the high current densities used in the die itself. As a result, the points where the leads penetrate the walls are particularly vulnerable to the formation of gaps through which gases can enter the package and contaminate the die.
The typical method of sealing the interfaces between the metal leads and the package walls is to form the package by molding the walls directly over the leads. Injection molding or transfer molding are typically used, starting with a molten heat-curable resin which is cured either during the molding process or shortly afterwards in a post-cure. These molding techniques form a mechanical bond between the metal and plastic which, for the reasons enumerated above, is less than fully effective for packages that are subjected to high current densities. Furthermore, in these processes the molding tool is typically controlled to a temperature in the range of 100.degree. C. to 175.degree. C., while the metal surfaces of the leads are considerably lower in temperature. The temperature difference inhibits the curing of the molten resin and further lessens the strength of the bond. An additional source of leakage arises from the fact that the typical resins used as molding compounds have adhesive properties themselves and therefore require release agents to prevent their adhesion to the molding tools. A typical release agent is a microcrystalline wax, which is incorporated into the molding compound formulation. The release agent unfortunately also weakens the bond between the molding compound and the metal leads.
One solution, which has not been previously used or disclosed to the knowledge of the inventors herein, might be to simply coat the entire lead frame with adhesive compound prior to molding the package body over the leads so that the adhesive would form a chemical bond between the leads and the molding compound when the latter is cured. A difficulty with this solution is the critical nature of the electrical connections that must be made between the leads and the die. Any contamination of the surfaces of the leads at the bonding location will interfere with the wire bonding process by which a reliable electrical connection is created. This can only be prevented by an expensive and difficult cleaning process after the molding operation has been completed. This problem is aggravated in leads that are plated with electrically conductive material such as silver or gold to enhance the electrical contact with the die.